The present invention relates to a method and a system for assisting a driver of a vehicle during operation by providing the driver with a desired steering feel.
It is known to provide the driver with a desired steering feel by applying a guiding force to the steering device based on different input parameters, such as steering device deflection, lateral acceleration, yaw rate and vehicle speed.
The guiding force exerted onto the steering device is resistive if counteracting the force applied by the driver onto the steering device, or supportive if acting in the same direction as the force applied by the driver onto the steering device, thus for instance reducing the effect of e.g. frictional forces acting on the wheels and the like which are experienced by the driver as resistance when operating the steering device.
The steering device is normally formed by a conventional steering wheel in the case of a vehicle. However, the invention is applicable to other steering devices, such as a joystick, a sliding nipple or any other suitable steering device for steering the vehicle. For instance, in the case that the steering device is a steering wheel, the guiding force will appear as a guiding torque exerted onto the steering wheel. Thus, in this case, the term steering feel denotes the steering wheel torque experienced by the driver during operation of the vehicle.
EP 1431160 discloses a system for estimating a steering wheel resist torque based on steering wheel rotation angle, vehicle speed and lateral acceleration or yaw rate. The steering wheel is connected to the road wheels via a steering shaft arrangement and the delivered steering wheel resist torque is measured and compared with the estimated steering wheel resist torque, whereupon the delivered steering wheel resist torque is adapted by use of a feedback controller to be substantially the same as the estimated steering wheel resist torque. It has turned out that this system in certain operations may be regarded as unsteady. Further, it has turned out that the steering feel is reduced when driving in long curves.
One object of the invention is to achieve a method for assisting a driver which creates conditions for an improved steering feel.
This object is achieved by the method defined in claim 1. Thus, it is achieved by the steps of:                determining a desired steering device guiding force comprising a part representing a desired friction in a steering arrangement, and        providing the driver with the desired steering feel based on the determined steering device guiding force.        
The invention is based on the insight that a certain friction feel is desired, see further below. Preferably, the desired steering device guiding force represents a nominal, desired friction. Said term “steering arrangement” may be a nominal steering arrangement.
The determined desired steering device guiding force can be based on further information, such as further desired steering characteristics in addition to the friction component. Thus, the determined desired steering device guiding force may be a sum of desired forces (such as torque components).
Preferably, the vehicle comprises an actual steering arrangement comprising a manual steering device, at least one pair of ground engaging members and a mechanical connection between the manual steering device and said ground engaging members.
According to an example embodiment, the method comprises the step of providing the driver with the desired steering feel by applying a final steering device guiding force based on the desired steering device guiding force to a manually operated steering device. Said manually operated steering device is preferably formed by a steering wheel. The term “final” with regard to the steering device guiding force defines in this case that the actively supplied steering device guiding force does not necessarily equal the determined desired steering device guiding force. For example, the method comprises the step of determining an actual force to the steering device resulting from an actual vehicle steering arrangement during operation, and determining a value of a final steering device guiding force to be applied to the steering device by subtracting a value of the determined actual force from a value of the determined desired steering device guiding force.
Preferably, the actual force is formed by an actual torque to the steering wheel, which is determined via an elastic element, such as a torsion bar, in the steering arrangement.
Preferably, a steering torque is determined by measuring the twist of the torsion bar in the steering arrangement. More precisely, a first angular sensor is arranged at a first end of the torsion bar and a second angular sensor is arranged at a second end of the torsion bar (opposite the first end). The steering torque can be determined based on the relative angular movement (twist) of the torsion bar and the stiffness of the torsion bar. According to an alternative, one or several strain gauges may be used.
According to an example embodiment, the method comprises the step of at least suppressing a driver steering feel from the influence of friction in a steering arrangement in the vehicle, which is configured to steer at least one ground engaging member.
The suppression of the driver steering feel from the influence of friction in the steering arrangement is preferably accomplished via a so-called reference generator function or any other known means of friction compensation device or function.
The suppression of the driver steering feel from the influence of friction in the steering arrangement is preferably accomplished simultaneously as the application of the steering device guiding force.
Preferably a driver steering feel is decoupled from the influence of friction in the steering arrangement. Thus, the influence of friction in the steering arrangement is preferably completely removed.
This embodiment creates conditions for using an Electrical Power Assisted Steering (EPAS) system. Especially, the method is applicable in steering systems where there is a mechanical connection between the steering device and the ground but where the inherent steering feel resulting from the mechanical connection during operation is eliminated or at least suppressed.
For example, the inherent mechanical friction in the actual steering arrangement depends on different operational conditions, such as manufacturing tolerances, wear, temperature, age etc. Thus, the mechanical friction in the mechanical connection is different for different individual vehicles and varies over time. Thus, this embodiment creates conditions for decoupling the hardware (mechanical connection) from the friction steering feel. In other words, the embodiment creates conditions for an application-independent (hardware-independent) friction steering feel.
Preferably, the method comprises the step of determining a desired steering device guiding force (based on a vehicle state, such as a steering angle. The steering angle is preferably defined by a steering wheel deflection.
Preferably, the method comprises the step of applying the determined steering device guiding force to the steering arrangement and simultaneously at least suppressing steering device disturbances resulting from the mechanical interconnection.
Preferably, a delivered steering device guiding force is measured and compared with an estimated desired steering device guiding force, wherein the delivered steering device guiding force is adapted by use of a feedback controller to be substantially the same as the desired steering device guiding force through adapting the amount of said guiding force.
According to a further example embodiment, the method comprises the step of determining the steering device guiding force by means of at least one predetermined friction model of the desired friction in the vehicle steering arrangement. Preferably, the method comprises the step of applying the steering device guiding force to the steering arrangement by means of an actuator.
According to a further example embodiment, the friction model represents the friction between a flexible element and the steering device in the steering arrangement, wherein the flexible element is configured for a relative displacement in a circumferential direction about a steering axis, which displacement is generated between an upper and a lower shaft of the steering arrangement. This embodiment is based on the insight that this specifically defined friction is advantageous for the steering feel.
The displacement in the circumferential direction is generated between the upper and the lower shaft of the according to turn of the steering wheel with twist of the flexible element (torsion bar) interposed between the upper and lower shafts.
Friction in the upper steering column is desired due to the fact that:
friction dissipates energy and thus adds damping (in a non-linear manner) to the system,
a certain amount of friction reduces the drivers torque in long curves, the driver can so to say “rest his/her arms in friction”, and
a steering system without friction is experienced as a pure spring.
According to a further example embodiment, the friction model is configured to not represent the friction between an actuator for applying the steering device guiding force to the steering device and the flexible element in the steering arrangement, but rather represent the desired amount of friction between steering wheel and the flexible device. This embodiment is based on the insight that only this specifically defined friction is advantageous for the steering feel.
Thus, friction in an electric power assisted steering system is distuingished into two separate categories, namely:                friction above the torsion bar, which (to a certain degree) is desired, and        friction between the electric actuator and the torsion bar, which is undesired from a control and stability point of view.        
A further object of the invention is to achieve a system for assisting a driver which creates conditions for an improved steering feel.
This object is achieved by the method defined in claim 11. Thus, it is achieved by a system for assisting a driver of a vehicle during operation by providing the driver with a desired steering feel, characterized by a means for determining a desired steering device guiding force comprising a part representing a desired friction in a steering arrangement, and means for providing the driver with the desired steering feel based on the determined steering device guiding force.
According to an example embodiment, the system comprises a means for suppressing a driver steering feel from the influence of friction in a steering arrangement in the vehicle, which is configured to steer at least one ground engaging member.
According to a further example embodiment, said means for determining a desired steering device guiding force comprises at least one predetermined friction model of the desired friction in the vehicle steering arrangement.
According to a further example embodiment, the friction model represents the friction between a flexible element and the steering device in the steering arrangement, wherein the flexible element is configured for a relative displacement in a circumferential direction about a steering axis, which displacement is generated between an upper and a lower shaft of the steering arrangement.
Further example embodiments and advantages thereof emerge from the description below, the figures and the claims.